1. Field of the Invention
The invention relates to a blade carrier for an axial-flow compressor, preferably a high-pressure compressor subjected to high thermal loading, the blade carrier being provided with cooling passages, through which a cooling medium flows in a closed circuit.
2. Discussion of Background
Cooled or heated blade carriers for turbomachines are sufficiently known. In order to solve the starting problems of a steam turbine, it is already known from BE-A 649 186 to arrange a system consisting of tubes, passages, lines and the like between the blade carrier and outer insulation and in a circular or spiral manner around the blade carrier in order to keep the latter at a desired temperature at all times by supplying external heat.
In axial compressors, in particular high-pressure compressors as well, as used, for example, in stationary gas turbines or turbine engines for compressing the combustion air, a radial clearance in the order of magnitude of 1 mm is provided between the outer ends of the moving blades and the inner wall of the compressor casing, which clearance is to be kept as small as possible so that the backflow of air and the reduction in the efficiency associated therewith can be kept slight. This correspondingly applies to the guide-blade tips, which provide a seal with respect to the rotor.
The reduction in the radial clearance is made more difficult by virtue of the fact that rotor blades and compressor casing expand or contract to a different degree in different operating states of the compressor. The radial clearance must therefore be selected in such a way that it is still adequate under the most unfavorable operating conditions, i.e., when rotor and moving blades have expanded and the compressor casing has contracted. It has to be taken into account here that the change in the radial clearance may have both mechanical and thermal causes. The mechanical cause may be in particular the radial deflection of the rotor and the moving blades caused by the centrifugal forces acting during high-speed rotation. Different thermal expansions in the rotor and stator on account of temperature differences or different coefficients of expansion of the materials used as well as the ovalization of the casing parts due to the joint in the parting plane may be regarded as thermal causes.
In the past, a multiplicity of proposals have been made which deal with the so-called active control of the radial clearance during operation. For this purpose, alternatively colder or warmer compressed air for example, which originates from different compression stages, can be directed into the interior of the rotor in order to control the radial clearance by controlling the temperature of the disks carrying the moving blades.
In addition to the abovementioned temperature control of the rotor, temperature control of the compressor casing has also already been proposed (U.S. Pat. No. 4,230,436), in which the temperature of the compressor casing is lowered in a controlled manner by a more or less intense cooling-air flow. In this case, the cooling air is bled at different compressor stages and passed along in cooling passages both behind the guide blades and behind the inner wall of the compressor casing opposite the moving blades.
The known methods for the active clearance control relate to the normal operation of the compressor. Therefore, for cooling or heating various compressor parts or sections, they may also resort to compressor air of varying temperature or--in the case of the compressor of a gas turbine--to hot gas from the engine part.
Here, the case of the so-called "hot start", during which the compressor starts again after a preceding shutdown but before it has completely cooled down, is not taken into account: in this case, the rotor and stator are at markedly different temperatures, since the outer stator cools down quicker and accordingly contracts, whereas the rotor remains hot for a longer period and accordingly retains its expansion. The radial clearance is thereby considerably reduced. So that renewed starting is possible in this state (hot start), this special case must be taken into account when dimensioning the radial clearance, a factor which leads to increased values of the radial clearance.